#2604: Suggest -XDeriveDataTypeable with "newtype" too
--------------------------------+-------------------------------------------
Reporter: kaol | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: trivial
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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I'm forwarding wishlist bug [http://bugs.debian.org/499216 #499216] from
Debian's BTS.
----
If I try to compile or load this program, ghc complains that I need
to pass -XDeriveDataTypeable to enable the derivation of Typeable
instances:
{{{
------ begin Test.hs --------
module Test where
import Data.Typeable
data List a = List [a] deriving(Typeable)
-----------------------------
}}}
{{{
Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(You need -XDeriveDataTypeable to derive an instance for this class)
In the data type declaration for `List'
}}}
If I use "newtype" and I have GeneralizedNewtypeDeriving turned on,
however, I get this error:
{{{
/tmp/Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(even with cunning newtype deriving:)
In the newtype declaration for `List'
}}}
It would be nice if ghc suggested DeriveDataTypeable here too. JFTR,
it does suggest this if newtype deriving is off:
{{{
/tmp/Test.hs:5:0:
Can't make a derived instance of `Typeable (List a)'
(You need -XDeriveDataTypeable to derive an instance for this class
Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving
extension)
In the newtype declaration for `List'
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2604>
GHC <http://www.haskell.org/ghc/>
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